Such a process is known from WO2006/105799. This document discloses a method for selective hydrogenation of acetylene to ethylene wherein a feed comprising acetylene and hydrogen is contacted with a TiO2 supported Group 10 metal catalyst in a fixed bed reactor, and wherein the catalyst is diluted with a inert solid like SiO2, ZrO2, Al2O3 or TiO2, preferably at a weight ratio of diluent to supported catalyst of from about 50 to about 170. The catalyst is reported to be suitable for converting acetylene in a gas stream that contains up to 20 mass % of acetylene.
Selective hydrogenation of alkynes to the corresponding alkenes is industrially important. Numerous documents focus on processes to remove acetylenic impurities, typically present in concentrations of 0.5-1 mol %, from ethylene.
Ethylene is a major intermediate for various chemicals, for example as a monomer that is used for the preparation of a large number of polymers. Ethylene is generally obtained by a hydrocarbon pyrolysis or steam cracking process, followed by separation steps. Polymer grade ethylene needs to be of high purity, and maximum allowable acetylene level is about 10 or even 5 ppm. One of the techniques used to free ethylene from any acetylene impurities is to selectively hydrogenate acetylene to ethylene over a palladium based catalyst supported on a suitable support such as alumina. A general problem with such metal based catalysts is that when the operating conditions are such as to permit complete elimination of the acetylene, a proportion of ethylene is also hydrogenated and converted to ethane. In addition, these single metal catalysts generally have relatively low stabilities due to the formation of a large amounts of oligomers formed, often referred to as green oil, which gradually cover the catalyst surface under operating conditions, thereby requiring frequent regeneration of the catalyst by some controlled oxidation process. A further disadvantage is that such catalyst may be quickly deactivated if used to hydrogenate acetylene in feed streams that contain relatively high amounts of acetylene, that is up to 10 or even 20 mass %.
Several other documents also address improving the performance of esp. Pd-based supported catalysts in hydrogenation of alkynes like acetylene. For example U.S. Pat. No. 5,648,576 discloses a process for selective hydrogenation in the gas phase of acetylene compounds containing 2 or 3 carbon atoms to the corresponding ethylene compounds in presence of hydrogen over a supported catalyst that have been surface modified with Pd and another metal from group IB, such as Ag, and optionally an alkaline or alkaline earth metal. Selective hydrogenation of acetylene is typically carried out on a feed containing 98% ethylene and 2% acetylene at a space velocity of 3300 h−1.
In U.S. Pat. No. 6,350,717B1 a catalyst comprising at least one Group 10 and at least one Group 11 metal in specific ratios on an alumina support is described, wherein the Group 10 metals are concentrated in a surface layer of the support. The catalyst were used in hydrogenating acetylene in a 1/99 mixture with ethylene.
WO03/106020 describes modification of a supported Pd—Ag catalyst with an iodide compound, to improve performance in hydrogenating acetylene in a mixed feed stream containing up to about 1.5 mass of acetylene.
U.S. Pat. No. 7,153,807B2 discloses supported Ni— or Pt catalyst further comprising at least two other elements selected from Groups 8-9 and Groups 11-12, which show better hydrogenation selectivity on feed streams comprising up to about 2 mass % of acetylene
U.S. Pat. No. 4,906,800 discloses a process for selectively converting a feed stream containing acetylene, ethylene and hydrogen into gasoline range hydrocarbons, wherein acetylene is hydrogenated by contacting with a Pd—Pb/CaCO3 catalyst that is made by a process including specific oxidizing/reducing pre-treatment steps. Such a catalyst would be suited for treating feed mixtures containing relatively high amounts of acetylene, like a pyrolysis effluent gas; exemplified by an experiment on a stream containing 2.6 mass % of acetylene.
EP0689872A1 discloses a catalyst comprising palladium, silver and a support material useful in a process for selective acetylene hydrogenation. Prior to use, the catalyst is contacted with a liquid reducing agent which inter alia may be boronhydride.
WO02/16032 discloses a process for selective acetylene hydrogenation using a catalyst comprising an inorganic support material, a palladium component, a silver component and a promoter component “XYFn,”, wherein said promoter component is a fluorine-comprising compound further consisting of an alkaline metal element and a further element which inter alia may be boron.
There remains a need in industry of a catalyst system showing high activity and selectivity, and good stability in hydrogenating alkynes in mixed feeds, also in feeds containing relatively high amounts of alkynes.